Ozonation, sonication, and their combination for petrochemical wastewater sludge pretreatment to enhance sludge disintegration

Large volumes of petrochemical waste-activated sludge (PWAS) generated from the oil refinery industry pose an immense risk for the environment and human health due to toxic compounds, high chemical oxygen demand (COD) and poor biodegradability. Anaerobic digestion to treat such sludge is limited due to the presence of recalcitrant compounds decelerating hydrolysis, and enlarging digester volume. Hence, the optimization of single ultrasound (US), single ozone (O3), and hybrid (combined US and O3) pretreatment methods based on sludge disintegration degree (DD) and the characterization of untreated and pretreated PWAS at the optimized process conditions were executed in this study. The highest DD of single US-pretreated PWAS was 72% at 20 kHz, 1.25 W/mL power density, and 61813 kJ/kg TS specific energy for 15 min sonication. The greatest DD of single O3-pretreated PWAS was recorded as 29% at 72.4 mg O3/min flow rate, and 0.157 g O3/g TS ozone dose for 15 min ozonation. When PWAS was pretreated with single US (1.25 W/mL for 15 min), single O3 (0.040 g O3/g TS for 5 min), and hybrid (US followed by O3, 1.25 W US/mL for 15 min, 0.040 g O3/g TS for 5 min) methods, and the most significant DD of 87% was achieved after hybrid pretreatment. Hybrid pretreatment displayed a synergistic effect on sludge disintegration which could be attributed to the contribution of ultrasonic cavitation on the disruption of sludge cell structure by ozonation owing to the amplified rate of mass transfer and ozone decomposition improving oxidation capacity of ozone.